I think that Zhongli is most aggravated by a lack of decorum or respect. That's why he struggles so much with Hu Tao - she's completely irreverent. He expects traditions, laws, and social contracts to be followed to the letter, so when a brash, reckless Snezhnayan diplomat comes to Liyue Harbor, flashing a too-wide smile and laughing at inappropriate moments, it gets on his nerves. Childe either doesn't know much about Liyuen customs or doesn't care, but either way he fails to show Zhongli, the Qixing, and the general populace the respect they are entitled to.

Later on, Childe's bold departure from tradition will seem endearing, commendable even. But that's when he's wearing the rose colored glasses of someone hopelessly in love.

this is a really good idea (though I didn't completely follow your prompt) but it sparked a lot of thought! sorry for the angst though!

Shortly after Zhongli’s retirement begins in earnest, the Eleventh Harbinger disappears. Not even Ningguang’s extensive network of spies can say where he is. It’s as though he’s vanished from Teyvat.

Zhongli worries. He spends his days going through the motions of his new mortal life, but always in the back of his mind is Childe, Childe, Childe.

One day the Traveler visits him, and shares a clue.

Aether’s directions take Zhongli to Konda Village, a quiet hamlet near Inazuma City where a young Snezhnayan by the name of Ajax has taken residence.

Ajax greets Zhongli with a smile that is friendly and nothing more. “Have we met?” he asks.

At his hip, his vision is gone. His shirt, simple spun wool, is buttoned all the way down and tucked conservatively into his pants. But that is not what stops Zhongli short. Not what leaves him open mouthed, stunned.

There is light in Ajax’s eyes.

His laugh is sweet as honey, lacking the dissonant, sinister note it once had. His smiles are pleasant, concealing nothing. His sharp edges are gone as though they never were, the darkness in his heart evaporated like dew.

He is no longer the man Zhongli knew, but he’s happy. He’s happy, and maybe that could be enough.

Zhongli lingers in the village, watching Ajax as he plays with children, fishes for his supper, and helps out in the fields. His blinding ambition, once bright as the sun, is entirely gone. In its place is simple contentment.

When the Traveler defeats the Shogun, Zhongli retrieves Ajax’s Vision from the statue. He feels the weight of it in his palm, and dreads the choice he must make. Is it merely selfishness to return it, to change simple, happy Ajax back to the vicious, ruthless man Zhongli loved?

Zhongli could never accept Ajax as he is, a man without a single scrap of malice, without the power and cunning he once possessed. Not when he knows and yearns for who Ajax used to be.

But Ajax is happy.

“You look like you’ve got something on your mind,” Ajax says to him, as evening falls over the quiet village and the Vision weighs heavy in Zhongli’s pocket.

Zhongli glances at him. “Are you happy, Ajax?”

“Sure,” Ajax says, with a friendly smile. “Are you, Zhongli?”

“I fear I am going to lose someone very dear to me.”

“I’m sorry. That sounds awful.” Ajax puts a gentle hand on Zhongli’s shoulder. “I’m here if you need to talk.”

“Thank you.” Zhongli brushes his fingers over Ajax’s. “It is well beyond time that I returned to Liyue. I wish you every happiness in the world.”

As he watches Zhongli leave, Ajax brushes his hand over his cheek and is surprised to find his face is wet. He doesn’t know why he’s crying, what deep sorrow within him broke open just briefly at Zhongli’s parting.

But it’s buried soon enough, and he returns to the warm lights of Konda Village where his humble life awaits.

The courtesan is here to assassinate him.

The lovely creature they call Childe is clever, but Emperor Morax has spies watching every move the Tsaritsa makes, and they whisper the truth to him before Childe first sets foot in Liyue.

Morax intends to slay him the moment he steps before the throne, robes the color of the sea falling around him and a sly smile on his lips.

But something stays his hand.

Perhaps the courtesan will be of use, will let some state secret slip as they while away the afternoons together, pretending at a more amorous relationship.

But as the days go by, it becomes clear Childe is too skilled a diplomat for any such mistakes. And yet Morax continues to seek his company.

Finally, Childe attempts seduction. Morax takes him to bed, pinning his hands above his head and keeping a careful eye out for the flash of the dagger he knows Childe has concealed on his person. But it never comes.

It would be foolish to sleep in the arms of an assassin, so Morax spends the night watching Childe sleep, petting his shock of orange hair and listening to the soft sighs he makes in his dreams.

In the morning, Childe wakes to the sight of his weapons and his correspondence with the Tsaritsa, laid out on the dresser. He sits up quickly, pulling the sheets to his chest as the combination of his bared skin and his predicament make him feel incredibly vulnerable.

He should have killed Morax last night when he had the chance. But he couldn’t bring himself to act. If he’s being honest with himself, he’s been in love with Morax for a long time now, a love an emperor could never return to a courtesan.

He thinks Morax will have him executed by sundown. But instead, Morax proposes a contract. A marriage, with a few concessions to the Tsaritsa’s demands as a dowry.

Despite the decorum that has been drilled into him since birth, Childe leaps into Morax’s arms and peppers him with quick kisses as he promises, over and over, to be true.

In a distant land, the Tsaritsa reads over Childe’s letter, pleased but not surprised. She had known Morax was lonely, and that Childe was clever and strong enough to be his perfect match and was in fact quite proud of herself for bringing their alliance about.

“Duke Morax is very, very wealthy and highly influential,” the Tsaritsa purrs, tucking a sparkling diamond pin into Signora’s hair. “When he marries Signora, it will be a great boon for our family.”

“If he marries Signora,” Scaramouche whispers to Childe. He’s lucky the Tsaritsa doesn’t overhear.

Childe just shrugs. It’s pretty likely Morax will fall at her feet like most men do. Childe is just along for the ride. He and Scaramouche aren’t even actual members of the family, but were adopted as boys to serve in the kitchens and later became enforcers and occasionally assassins.

It’s a pretty far drop in status from the Tsaritsa’s beautiful daughter Signora, but he doesn’t care. Nobles spend all their time scheming and bickering anyway. He’s glad to have a job that keeps him active, that involves danger and violence.

Signora doesn’t say much on the carriage ride over. Childe knows she’s not happy about this; she doesn’t want to get married but she’ll do what’s best for the family.

When they get there, Scaramouche draws the short straw and has to wait in the stuffy drawing rooms with Signora until the Duke arrives. Freed momentarily from bodyguard duties, Childe wanders the spacious grounds, marveling at the Duke’s wealth.

The gardens are divided by thick hedges and carpeted by lush green grass. Childe follows the stone path to the shore of a lake. The water is clear and cool, enticing on a sweltering summer day like today.

Childe has never been good at propriety, or following the rules. And he really, really likes to swim. After a quick glance around, he strips down to his underclothes and dives in, letting the refreshing water wash away the grime and weariness of the long journey.

But when he comes back up for air, someone is standing by his pile of discarded clothes. The man is tall, and possesses a stern dignity that belies his youthful appearance. He is also ridiculously handsome, and despite the chilled water, Childe’s face flushes as he makes his way back to shore.

He doesn’t miss the elegant gold fastenings on the man’s clothes, the quality of the cloth and the rings on his fingers. This has to be the Duke. Childe is standing, nearly naked and dripping wet, in front of the guy Signora is supposed to convince to marry her.

Well, shit. Nothing to do but roll with it. He gives Duke Morax his most charming smile and laughs like he doesn’t have a care in the world.

“Your lake is so nice, I couldn’t help myself. I have to thank you for your hospitality.”

The Duke’s eyes linger on Childe, dipping to take in his muscular thighs, then running upward over his chest, until the Duke breaks his gaze away with some dark, unfathomable emotion in his eyes.

“The Tsaritsa sent me.” Childe wants to reach for his clothes, but he would have to bend over and get intimately close to the Duke to get them.

“Ah,” Morax says. “She has indeed been very persistent in bringing us together. Perhaps I was wrong to dismiss her advice.”

“Huh?”

“Surely you know your mother has been most eager to see me take a bride.”

Well. They say Duke Morax isn’t one to beat around the bush, but prefers a direct approach. And this is about as direct as it gets.

“Perhaps it is time that I should.” Morax bows deeply, a slight smile on his face. “It is a pleasure to meet you, Signora. You are as beautiful as was promised.”

Childe squeaks, then flushes red. “I… uh. Thanks. But I’m not Signora. I’m just the bodyguard.”

“Hmm. But you are a treasure. Will you let me court you, bodyguard?”

The Tsaritsa is going to be pissed, but there is no way Childe could refuse. Morax’s very presence is intoxicating, his voice rich and resonant and his eyes gleaming like true gold.

After that, it’s just like a story. They talk late into the evening, sitting on a bench near the lake. And when Childe kisses him just as the stars come out, Morax pulls him into his arms like he’s always belonged there.

They make love in the gardens on the rich, thick grass, so wrapped up in each other they forget where they are, or what time it is, or the fact that Signora and Scaramouche are still in the Duke’s sitting room, waiting politely for the Duke to come meet them.

Childe’s heart hammers in his chest, struggling to breathe. He’s lost in the woods, running for his life from some great evil that draws ever closer. The light that filters between the leaves of the trees fades away, and he’s trapped in a great maze of darkness that more and more begins to resemble the everchanging passageways of the Abyss.

He wakes up screaming, a blade of glowing blue Hydro nearly through the heart of the Fatui agent who dared to come check on him.

The next time he has a nightmare, they let him be. And when he wakes up, his cheeks are wet and his throat is hoarse from crying out in fear.

He gets used to it; he adapts. He sleeps less, spends his nights training or studying up on Liyuen culture. Harbingers are known for their eccentricity, and his staff quickly grow accustomed to his insomnia.

The first night he spends in Zhongli’s bed is a revelation.

He isn’t sure if he even trusts the consultant, but his subconscious must, because after they fuck he falls asleep before Zhongli can even ask if he wants to stay the night. And he sleeps until well past dawn, waking to an unfamiliar comfort and warmth—Zhongli’s arms around his waist.

He can’t get enough of this new thing. Not just the sex (which is amazing) or Zhongli’s company (which makes his heart do funny flips and jolts in his chest), but also the deep, dreamless sleep.

And then Osial. And then the Traveler atop the Jade Chamber, shattering the plan he worked so hard to put into place. And then the revelation: Zhongli, the Geo Archon, who had been toying with him from day one. After that he’s back to his bed in the Fatui compound, waking up with the foul taste of the Abyss on his lips, drowning in his fear as soon as he closes his eyes.

One day, he finds a brown dragon in the Abyss, surrounded by a soft glow like the radiance of crystals deep underground. In his blind panic, Childe runs nearly into its lithe, serpentine body. The dragon curls around him protectively, teeth bared to fight back the horrors that relentlessly chase him.

“Is this what your dreams are like?” The dragon’s voice is very familiar. “Had I known, I would have interceeded sooner. Rest now, my love. I will watch over you.”

After that, Childe slips into a deep, tranquil sleep. When he wakes, Zhongli is at his door with a bouquet of glaze lilies and the promise of a real spar.

Childe forgives him. How could he not?

From then on, his sleep is peaceful and untroubled. Curled in Zhongli’s arms, he feels more safe and secure than ever, and his dreams are guarded by a dragon who looks suspiciously like Rex Lapis and who brings warmth and light into even the darkest, coldest part of Childe’s psyche.

Childe learns the hard way not to get embarrassed about stuff. Because when he gets embarrassed—like really, truly embarrassed—he shifts. Into the shape of a small fox.

He can’t control it. Archons, how he’s tried. But by the time he was in Fatui basic training, he realized the only thing to do was become immune to embarrassment.

It’s served him well throughout his life. He can nonchalantly laugh off mistakes that would have others cringing, and he has a swagger and easy confidence that comes from being notoriously hard to ruffle.

He’s fine. He’s managing. The last time he turned into a fox was almost a year ago, and there was no one around but Teucer.

He’s got it under control. Until Mr. Zhongli hands him a pair of chopsticks.

He likes Zhongli a lot. It’s not a crush, he tells himself, but he does want to impress the consultant. So he takes the chopsticks with all the confidence of a man who mastered four weapon types by the time he was twenty. After some instruction and careful finger positioning, he picks up a crystal shrimp, lifting it to his mouth.

It falls directly in his lap, and the corner of Zhongli’s mouth quirks up just slightly.

Childe’s cheeks burn and he feels the hot, prickly embarrassment take him over. He stands abruptly, banging his knee on the table, to make a quick exit. But it’s not fast enough. He lands on the floor on four paws and looks up at Zhongli, who is suddenly much, much taller.

“Now now,” Zhongli says, low and soothing. “There’s no need to skulk about. Come up here and I’ll feed you.”

He pats his thigh and without thinking, Childe leaps up into his lap. It’s all instinct—Zhongli’s voice is just that naturally commanding. That’s it. That’s all it is.

Zhongli feeds Childe his lunch, scratching behind his ears and continuing his lecture on the history of the tableware they’re eating off of. It lulls Childe into an easy calm, until the embarrassment eases away and he shifts back without meaning to, leaving Zhongli with a lap full of Fatui Harbinger.

After that, it gets a little easier. Childe knows that if he accidentally shifts, he can find Zhongli at the funeral parlor or the market or his home and Zhongli will look after him until he shifts back. He takes advantage of Zhongli’s kindness as often as he can, and Zhongli doesn’t seem to mind.

The first time he tries to kiss Zhongli, he moves at the wrong moment and their foreheads clunk together instead. With a desperate yelp, Childe shrinks down to his fox size and remains that way for half a day. Zhongli kisses him on his little fox snout and promises to give another kiss when Childe shifts back.

From the Comprehensive Dictionary of the Teyvat Common Tongue

noun: catalyst; plural noun: catalysts

• a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.
  "Hydro acts as a catalyst promoting the vaporization of Pyro and the amplification of elemental techniques"

• a person or thing that precipitates an event.
  "the Snezhnayan diplomat’s arrival acted as a catalyst for the Osial incident"

• a weapon type that allows a Vision user to perform elemental attacks and amplify their elemental abilities
  “the power of his giant Hydro whale was increased by 150% when wielding a catalyst”

Childe wakes up chilled and disoriented, his skin clammy and his limbs leaden. He struggles to sit up and a firm hand lands on his shoulder, warm press of fingers on dry skin, pushing him down.

“Please rest.”

The voice is low and resonant, thrumming through the dimly lit room. Childe is lying on some kind of cold stone table, and around him he can see…

“Shit,” he hisses, scrambling again to sit up. “Are those coffins?”

“Calm yourself, Harbinger Tartaglia. You collapsed on the street nearby. Heatstroke, I believe. I brought you here to Wangsheng Funeral Parlor to recover.”

Wangsheng… Childe blinks away the blurriness in his eyes and turns. The man by his side is tall and elegant, with golden eyes that seem to glow in the shadows.

“Then you must be Mr. Zhongli.” Childe’s head feels heavy, his arms and legs weighted, but he’s had worse. “I’ll admit, this isn’t how I wanted to make your acquaintance. Guess I’m just not used to Liyue’s heat.”

With more effort than he would like, Childe manages to heave himself off what looks alarmingly like an embalming table and stand mostly upright.

“I have to thank you for your assistance, Mr. Zhongli. I’ve been hoping to get in touch with you. What do you say we do lunch tomorrow?”

Zhongli smiles, a slight, restrained curve of his lips. “It would be my pleasure.”

—
“Aiyah, who was that?” Hu Tao pokes Zhongli on the shoulder as he lingers in the doorway, watching Childe disappear into the busy street.

A child of the Abyss, he does not say. A foreign agent, sent to bring chaos to Liyue. A charming diplomat, hard to read but easy to like.

He can’t help but look forward to their next lunch, and to the turbulence Childe is certain to bring to his ordinary mortal life.

The child looks like a miniature version of Childe, complete with the shock of orange hair and the ocean-blue eyes. He scowls at Zhongli, arms crossed.

“I won’t talk to that guy,” he says to Childe. “Tell him to go away.”

“Why not?” Childe asks.

“He’s the old dragon who was mean to Big Brother.” Teucer speaks with righteous indignation. “I don’t wanna even look at him.”

Zhongli can’t help but be dismayed. This is not the first impression he wanted to make.

“It’s okay though.” Childe pats Teucer’s shoulder. “Sure, he lied to me. A lot. But he had to do it for his work. I’m not mad at him for being a self-centered old dragon.”

Zhongli can’t help but feel that Childe is, in fact, a little bit mad.

“It’s fine,” Childe says. “It’s totally fine. I’m not angry. Not even a tiny bit.”

“Oh.” Teucer shuffles his feet, unprepared for this onslaught of adult emotions.

“That’s just what people do. They betray each other. Like when Mom had that friend sleep over all the time, and then Dad got mad when he got back from his trip.”

Zhongli steps forward. “This is nothing like that. I would never betray your trust like that.”

Childe doesn’t look at him. “C’mon Teucer. Let’s go fly kites.”

“I will make you a contract,” Zhongli says, desperate to push through the wall of ice Childe has erected between them. “I offer you my love and honesty, for as long as either of us shall life.”

“Big Brother.” Teucer tugs eagerly at Childe’s hand. “That fixes it, right? In school we learned that people in Liyue can never break a contract or else Rex Lapis drops a big rock on their heads.”

Zhongli winced at the mangling of his culture, but kept his focus on Childe.

“What do you want in return?” Childe asks.

“Only that you return to me, as often as you can.”

At that, Childe gives him a real smile, the kind he’s not seen since Osial’s attack. “I promise,” he says.

Teucer puts his hands on his hips. “Pinky promise?”

Childe holds out his hand, and Zhongli interlocks their smallest fingers. “Pinky promise,” he vows.

This is an excellent one!! I love jealousy for this ship <3 Here's a little something:
“Does she have to do that?” Childe asks, then cringes, realizing too late the harsh tone he’s taken.

Zhongli glances at him, confused. “Miss Xinyan was merely expressing her gratitude for my support.”

Childe huffs and turns away. He feels hot and prickly and all out of sorts. It’s unnerving how easily Zhongli (and the many, many people who constantly flirt with him) can get under Childe’s skin.

It shouldn’t matter. Zhongli fucks him, but that’s all it is. Childe doesn’t want anything more. (He does. He desperately does.) And Zhongli has made it clear he’s not expecting commitment.

“Is this a Snezhnayan thing, perhaps?” Zhongli asks. “To disapprove so strongly of a little flirtation?”

Childe rolls his eyes. Zhongli probably understands Snezhnayan customs better than Childe himself, given his age and his love for culture and history.

“No?” Zhongli looks amused. It’s both endearing and annoying. “Then what?”

“It’s nothing.”

Childe turns away, taking a step towards the door, but Zhongli catches his arm and pulls him back. Fixes him with that steady golden gaze, pins him in place to watch him squirm.

“You’re not jealous, are you?” All pretense aside, Zhongli’s eyes are narrow and intense, his mouth a firm line. “You cannot possibly be jealous when you have sworn yourself, body and soul, to another.”

Childe opens his mouth, then closes it. Zhongli’s voice has taken on a divine resonance, thrumming through Childe’s bones, and his eyes are glowing.

“Do you know how it eats at me? To know you would offer yourself first to the Cryo Archon, and reserve only the scraps she does not find useful to give to me. You worship another, and yet you complain to me because some sweet young woman touched me on the arm when she thanked me for my help.”

His heart thudding in his throat, Childe takes a step forward, then drops to one knee, looking up at Zhongli.

“I would give myself to Rex Lapis,” he whispers reverently. “If he would have me. If he would risk the Cryo Archon’s wrath, and take a child of the Abyss into his fold.”

Zhongli’s fingertips brush Childe’s forehead, and it feels like a blessing, a kiss of the divine.

“I promise to cherish and love you, to provide for you, to protect you,” Zhongli says. “This is my contract, and it is as solid as stone.”

“You know, I could set my watch by you,” Childe said, placing a bowl of slow cooked bamboo shoot soup in front of his most reliable and constant customer.

“Is that so?” Zhongli looked up at him from his place at the bar, which faced the large window leading into the kitchen. The perfect place for Childe to lean on the ledge where he set food orders and chat with the knowledgeable consultant, because Sunday afternoons were slow and sometimes Zhongli was his only customer.

“Every Sunday, two pm,” Childe said. “I don’t even have to take your order at this point.”

Zhongli dipped a spoon into his soup, humming thoughtfully. “I suppose I have become predictable.”

“That’s fine, but is there something special about 2 pm at the Third Round Knockout?”

“There is.” Zhongli took a small bite of the soup, nodding his approval at the taste. “You are the only chef who can prepare this dish to perfection. I prefer the quiet, so I come at this hour. In addition, you are good company and pleasing to look at. I enjoy watching you cook.”

Childe made a slight choking noise, heat rushing to his cheeks, but after a moment he rallied. If this was his chance to get with the most attractive man in Liyue Harbor, he wasn’t going to mess it up.

“I’d bet I’d look even better cooking in your kitchen,” he said with a wink and a flirtatious grin.

“I have thought of that,” Zhongli answered gravely. “The slow cooked bamboo shoot soup takes approximately six hours to prepare, but much of that is waiting for the broth to absorb the proper flavor.”

“Trust me,” Childe said, leaning his hip against the table and tugging gently on Zhongli’s tie. Never let it be said that the best chef at Third Round Knockout lacked for nerve. “I could think of something for us to do.”